1. Field of the Invention
The present invention relates to the preparation of rare earth borides.
By the expression "rare earth" according to this invention are intended the rare earth elements designated the lanthanides, having atomic numbers ranging from 57 to 71, inclusive, and yttrium, having an atomic number of 39.
2. Description of the Prior Art
The rare earth borides are known compounds having desirable electrical properties In particular, lanthanum hexaboride has excellent properties of thermionic emission and is used as an emissive cathode in high energy electrical devices.
At the present time, the industrial development of the rare earth borides is limited by the difficulty of producing a product of satisfactory purity by a process applicable on an industrial scale.
It is known to this art to prepare lanthanum boride by reducing lanthanum sesquioxide with boron carbide or elemental boron under reduced pressure (G. A. Meerson et al, Izv. Akad. Nauk. SSR Neorg. Mater, 3, No. 5, 802-806 (1967)). However, as these reactions are carried out at temperatures higher than 1,500.degree. C., not only is a graphite crucible required, but also a graphite lined furnace.
It has also been proposed, according to U.S. Pat. No. 3,902,973, to prepare rare earth borides by the electrolysis of a source of rare earths in a molten salt medium containing a cryolite and an alkali metal borate. Such a process is relatively complex in view of the electrolysis temperature of from 950.degree. to 1,050.degree. C. and the usual problem of the recovery of the rare earth boride at the cathode arises, with the latter also causing pollution because of the cryolite.
In the process described in U.S. Pat. No. 4,260,525, the same problem of separation of the rare earth boride is encountered. This process entails mixing a rare earth carbonate, nitrate or oxide with boron, in the presence of aluminum, to dissolve the boron at a temperature of from 1,200.degree. to 1,600.degree. C., next heating the reaction medium within this temperature range, cooling it, and then separating the rare earth boride from the aluminum oxide formed.
To eliminate the difficulties of separation and purification of the products produced by the above processes of the prior art, the assignee hereof, in FR 87/16,396, proposed a process for the preparation of a rare earth boride by heating a rare earth chloride together with elemental boron.
The process of this '396 application features directly producing a rare earth boride by heating a rare earth chloride with elemental boron, as the only secondary product formed is boron chloride, which is volatile under the conditions of the reaction.
Another advantage of the process described in FR 87/16,396 is that it can be carried out in conventional apparatus, e.g., in a furnace of alumina or aluminosilicate bricks, as the reaction temperature is relatively low; it may be less than 1,500.degree. C. and preferably around 1,200.degree. C.
As the reactions for the synthesis of the rare earth borides are carried out at a relatively high temperature, need continues to exist in this art for processes which can be conducted at even lower reaction temperature.